Processing of raw cellulosic materials



Patented Jan. 6, 1931 UNITED STATES v PATENT OFFICE GEORGE A. RICHTER, OF BERLIN, NEW'HAIIPSHIRE, ,ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAlI PSHIRE, A CORPORATION OF MAINE rnocnssme or RAW CELLULQSIC MATERIALS No Drawing.

is cooked in an acid liquor and the liquor is then converted into an alkaline liquor by the addition of alkali and cooking is then carried out in the alkaline liquor, it is possible so to control the compositions of the acid and alkaline liquors and the time, temperature and other conditions of cooking as to produce a finished pulp of high alpha cellulose content.

In accordance with the process of the present invention, the initial step of cooking is carried out in an acid liquor short of complete fiber liberation, and ,the subsequent step of cooking is carried out inan alkaline liquor,

. the alkalinity of which is preferably afiorded at least in part by sodium sulphide. By terminating the acid cook short of complete fiber liberation, it is possible to avoid the loss in yield'and inju to the fibers occurrin when \the isolated bers are digested in am liquors. By carrying out the second cook ing step in an alkaline liqubr contalnmg sodium sulphide, not only is itpossible to complete fiber liberation and to produce a finished pulp of high alpha cellulose content',.but, because of the mild action of the'sodium sulphide on the fibers as compared with alkalies such as caustic soda, it is possible to produce a product having physical characteristics superior to those possessed by products-produced by cooking in alkaline liquorsin which caustic soda alone is used as a-fiber-hberating chemical. 7

Appli cation filed September 24, 1929. Serial No. 394,930.

tions may be employed in carrying out the initial cooking step, depending upon the characteristics desired in the finished product. When a finished pulp especially low in pentosans is desired, the cooking liquor is preferably one containing sulphurous acid alone or together with another acid reagent such as sulphuric acid. Evidently such a liquor effects more pronounced hydrol sis of the pentosans present in the raw ce ulosic material, producing water-soluble reaction products. A finished pulp of low pentosan content is especiallydesirable when it is to be used 'as a raw material for the production of cellulose derivatives such as-the acetate, as pentosans appear to impart undesirable characteristics such as opaqueness and cloudiness to lacquers or films produced from the derivative. If, however, the finished pulp is to be used as a raw material for papermaking, it may be preferable toemploy an acid cooking liquor containing SO combined in the formof sodium sulphite or containing sodium saltssuch as sodium isulphate, sodium sulphate, or sodium borate, which undergo metathesis in a sulphurous acid solution to give sodium sulphite. After cooking in the sulphurous acid liquor to the desiredv degree, it is preferable to release or exhaust the free S0 content of the hot spent liquor, so that alkali need not be consumed in react- .in the subsequent step of alkaline cooking.

The spent acid liquorv contains residual sulphurous acid groups existing as sulphurous acid or combined in the form of loose,

complex organic compounds. If sodium sul-.

phide be added to the hot charge in the digester, not only is neutralization of the free acid effected, but reaction takes place to produce sodium thiosulphate and free sulphur. Some of the free sulphur may redissolve in thealkaline liquor to produce sodium thiosulphate, but this compound may decompose phurous acid groups are preferablyoxidized to sulphuric acid or sodium sulphate by a suitable oxidizing agent and the addition of alkali. This oxidation may be efiected to advantage when only caustic soda is to be added to produce an alkaline liquor, although it is particularly advantageous when alkali including sodium sulphide is added, since, aspreviously stated, a better quality pulp is produced in an alkaline cooking liquor containing sodium sulphide, and at the same time the formation of sulphites, thiosulphates, and sulphur is avoided by reaction of the sodium sulphide with the sulphurous acid groups. Various oxidizing agents, such as sodium hypochlorite, sodium peroxide, sodium perborate, or chlorine water, may be employed, oxidation occurring rapidly by the use of such agents and resulting in the-conversion of the sulphurous acid groups to sulphuric acid or sodium sulphate. It is also possible to efiect oxidation by the use of oxygen or air, under pressure, although such oxidation takes place much more slowly than when water-soluble oxidizing chemicals are used. After oxidation has been effected, alkali may be added tothe digester in the form of a solution of caustic soda or a mixture of caustic soda and sodium sulphide to produce an alkaline liquor, whereupon cooking may be con tinued in the resulting alkaline liquor until fiber liberation has been completed. The alkaline liquor in which fiber liberation is completed may be of sufliciently high alkalinity to produce a finished pulp of high alpha cellulose content, say 92% to 96%, or even higher, although, if desired the alkalinity of the liquor may be lower, such as to produce a pulp having an alpha cellulose content of, say, only 89% to 90%.

A process such as described makes possible the realization of many advantages, among which may be enumerated the following:

1. It permits the production of a finished pul of the desired alpha cellulose content witE a comparatively small amount of sodium base chemicals, as the initial step of cooking in an acid liquor may be carried out in a liquor containing little, if any, sodium base compounds.v Subsequent alkaline cooking may be effected in an alkaline liquor of a sodium base compound concentration not in excess of that employed in the usual kraft process, and often appreciably lower.

2. It makes possible the conservation of the heatassociated with the spent acid liquor and cellulosic material after cooking.

3. The step of washing the cellulosic ma- I terial after acid digestion may be avoided,

the same apparatus being used from the start -to the finish of the pulping operation.

4. The oxidation of the spent acid liquor permits cooking in a sodium-sulphide-containing alkaline liquor and results in a fin.- ished pulp having excellent physical and papermaking characteristics.

5. The entire process need require no more time than that of the usual sulphite process, as each of the cooking steps is of-s hort du ration, compared with the usual sulphite process, for example.

\Vhile various specific procedures'may be adopted in accordance with the present invention, the following example will serve as an illustration of a procedure intended for the production of finished pulp of high' alpha cellulose but low pentosan content. Any suitable raw cellulosic material, such as spruce chips, may be placed in the usual acid sulphite digester, together with a sulphurous acid. solution containing, say, 5% free S0 The digester charge may be confiued and slowly heated to a temperature of about 250 'F., say in about two hours, at which temperature it may be maintained for a period of about two hours, whereupon the temperaphurolis acid groups in the liquor. After the release of sulphur dioxide from the liquor, a suitable oxidizing agent such as sodium hypochlorite may be introduced into the digester. The amount of sodium hydrochlorite introduced is based upon the amount of sulphurous acid groups present in the liquor in the digester, this being ascertained by analyzing a sample of the liquor. The sodium hypochlorite in the form of a strong solution may be introduced near the bottom of the digester, so that natural circulation of the liquor will result in a diffusion of the solution through t-heliquor, although if indirect cooking is being practiced, the liquor may be pumped from the bottom of the digester, through the liquor heater, to the top of the digester, to ensure a-uniform diflusion of the oxidizing agent through the liquor. Any oxygen gas generated during the oxidizing reaction may be allowed to escape from the digester through asuitable relief valve.

After the oxidizing reaction has been completed, the desired amount of alkali may be added to the digester, as in the form of a the digester may vary, depending upon the alpha cellulose content desired in the finished pulp. If a finished product of, say, about 92% alpha cellulose content is desired, suflicient alkaline solutionshould be" added to the digester to produce a liquor containing 1.5 pounds of active Na O per cubic foot. If, however, a product of unusually high alpha cellulose content is desired, the alkali concentration produced in the liquor may correspondto 2.5 pounds of active Na O per cubic foot. Preferably about 40% or more of the alkali exists in the form of sodium sulphide, as the presence of sodium sulphide is advan tageous, particularly in the production of a pulp which is to be finally bleached and employed in papermaking. After the addition of alkali the liquor. contains caustic soda. sodium sulphide, sodium sulphate, and pos-' sibly slight residues of sodium sulphite. The

hot liquor .together, with the previously cooked chips may then be heated to a temperature of about 335 F. and maintained at this temperature for two orthree hours to complete fiber liberation, but this temperature may be varied somewhat, depending upon factors suchas the period of cooking and the alkali content of the liquor.

The hot digester contents may be blown as usual from the digester intoa pit,,and the pulp separated from the spent cooking liquor as by washing. The spent cooking liquor may then be concentrated to the desired consistency, as in multiple effect evaporators, and the concentrated liquor then delivered into a furnace, where the organic combustible material is burned and the inorganic sodium constituentvis smelted preferably under re-' ducing conditions to produce a smeltconsisting of sodium sulphide and sodium carbo-- nate. T he smelted compounds may then be dissolved in water to produce a solution which may be causticized with lime to produce an alkaline liquor containing caustic soda and sodium sulphide. The causticized liquor may be used in rendering the spent acid cooking liquor alkaline,

as previously described. The washed pulp may .be bleached, if desired, to produce a white product suitable for use as a raw material either for pa ermaking or for conversion into cellulose erivatives.

If the acid cooking stephas been carried out in a liquor containing sodium compounds, it will be seen that the sodium constituent present in both the acid cooking step and in the alkaline cookingstep is accumulated in a single aqueous liquor, from which recovery is efi'ected. When the oxidizing agent is asodium compound, sulphurous acid residues remaining in'the spent cooking liquor are changed intosodium-sulphur compounds which, afte r smelting, appear in the smelt as sodium sul hide, thereby resulting in'the recovery 0 sulphur constituent, which is ordinarily lost in the wateremployed for washing cellulosic material after acid su1-' John '1. Quinn, a higher fuel value than ordinarily is realized.

I claim:

1. A process which comprises digesting raw cellulosic material at elevated temperature in a sulphurous acid liquor, oxidizing residual sulphurousacid groups in the liquor,

adding suificient alkali to the liquor to render it alkaline, and digesting at elevated temperature in the alkaline liquor.

2. A process which comprises digesting raw cellulosic material at elevated temperature in a sulphurous acid liquor short of .fiber liberation, adding alkali including sodium sulphide in amount sufiicient to renderflthe liquor alkaline, and digesting at elevated temperature in the alkaline liquor until fiber liberation is complete.

3. A process which comprises digesting raw cellulosic material at elevated temperature in a sulphurous acid liquor, releasing sulphurousacid from the hot liquor, oxidizing residual sulphurous acid groups in such liquor, adding alkali including sodium sulphide in amount suflicient to render such liquor alkaline, and digesting in the alkaline liquor.

4.. A process which comprises digesting raw cellulosic material at elevated temperature in'a sodium base acid sulphite liquor short of complete fiber liberation, adding a sodium base oxidizing agent to the hot liquor to oxidize sulphurous acid groups therein, adding sodium base alkaline compounds includingsodium sulphide in amount sufficient to render the liquor distinctly alkaline, and digesting at elevated temperature in the alkaline liquor until fiber liberation is complete.

- 5. A process which comprises the following steps: (a) digesting rawcellulosic material, at elevated temperature in a sulphurouslacid liquor; (6) adding a sodium baseoxidizing agent to the hot liquor; (0) adding sodium base alkaline compounds thereto to render the liquor alkaline; (d) digesting at elevated temperature in the alkaline liquor; (e) separating the resulting pulp from the spent liquor; (f) and recovering the inorganic content of the spent liquor 1n the form of alkaline compounds for use in again carrying out step (c).

6. A process which comprises the follow.- ing steps: (a) digesting raw cellulosic material at elevated. temperature in a sulphurous acid liquor; (6) adding a. sodium base oxidizing agent to the hot liquor; (0) adding sodium base alkaline compounds including caustic soda and sodium sulphide thereto to render the liquor alkaline; digesting at elevated temperature in the alkaline liquor; (e') separating the resulting pulp from the spent liquor; (f) smelting the inorganic content of the spent liquor in a reducing atmosphere; (g) and causticizing the smelted compounds for use in again carrying out step (0).

In testimony whereof I have afiixed my signature.

GEORGE A. RICHTER. 

